Generally, very small telephone systems were generally referred to as key telephone systems. Characteristically, a key telephone system was provided by extensive telephone line and control lead wiring between the key telephone sets with each telephone line extending to a telephone switching office. Each of the key sets included a plurality of interlocking pushbuttons or keys each for connecting the telephone set to a particular telephone line among a plurality of telephone lines routed to each key telephone set. The switching function of line selection was mechanically provided and distributed among the key telephone sets. The primary advantages of these systems were economy with small size and robustness. However, these early systems were difficult to expand, required extensive cabling and were not compatible with the emerging world of digital switching and the associated ubiquitousnous of digital communications.
In the late 1980's and 1990's, a new generation of small switching systems were developed to be compatible with the existing world of digital telephony. One of the better-known and most successful of these systems is marketed under the name of Norstar.TM. by the Northern Telecom Company.
Architectural and functional information on the Norstar digital key telephone system may be obtained from U.S. Pat. No. 4,873,682 issued Oct. 10, 1989 to G. F. Irwin et al and U.S. Pat. No. 5,027,345 issued Jun. 25, 1991 to T. J. Littlewood; both patents are assigned to the same assignee as the present application and are hereby incorporated by reference. Yet other information on the Norstar system may be obtained from the publication "Telesis", 1989, two, published by Nortel Technology, the research arm of Northern Telecom Limited.
The Norstar system interfaces to the public switched telephone network (PSTN) via telephone lines in the usual manner of a telephone switch. The core of the system named KSU (Key Service Unit) in Norstar, has ports connected to telephone lines and to proprietary digital key sets via time-compression multiplex (TCM) links each carrying a pair of B-channels B1 and B2 at 64 kb/s and a D-channel at 16 kb/s. The telephone lines extending from the key system to a central office are often referred to as trunks since they connect two switches; therefore, the terms line and trunk are often used interchangeably in describing key systems.
The KSU can also be connected to a personal computer (PC) via one of its TCM ports. The physical and logical connection between the PC and the KSU is provided by a proprietary PC interface card that occupies one standard PC bus slot. Firmware in the PC interface card provides PC applications with access to both D-channel functional messaging and B-channel data. The PC communicates with the card via interrupts for the D-channel and direct memory access for large data transfers associated with the B-channel traffic.
By comparison with the electromechanical key switching systems, the new digital key switching systems are very sophisticated. However, a characteristic shared by both old and new systems presents a user with the opportunity of using a key station set fraudulently. The characteristic relates to the fact that when a user actuates a key on his set to request a line to a subtending central office (CO) of the PSTN, he causes the CO to allocate a subscriber line to be connected directly between itself and the key set. In the case of the Norstar system, one of the B-channels between the key set and the KSU and hence to the CO is allocated to the request and the user is effectively connected directly to a line circuit of the CO. This line seizure causes a DTMF (Dual Tone Multifrequency) receiver to be connected to the line appearance of the CO in anticipation of receiving dialed digits from the user. This situation allows a user who wishes to defraud the system to use a hand-held DTMF generator to dial digits for access to a remote location. This activity appears like a valid sequence to the CO and the user is able to remain connected for an unlimited duration and the KSU is only aware that the line has been successfully seized.
As mentioned above, contemporary key switching systems have become sophisticated and although they suffer from the key system characteristic described above, they, on the other hand, provide safeguards against misdialing and invalid dialing of destination numbers. This is achieved through the use of a restriction server which causes software routines to verify a dialing string of digits or requested feature codes against pre-programmed restrictions and exceptions. The restriction server parses the string of digits one digit at a time and the string is compared against all restrictions. If any one of the restrictions is failed, the parser will compare the current string against the corresponding exceptions. If no exception matches the string, restriction is considered as being failed and a functional message will be sent to the controller of the process to indicate this result. However, if the string of digits does not fail any of the restriction filters, then restrictions are considered passed and a corresponding message is generated.
Key systems are vulnerable to toll frauds partly because a restriction server normally assumes that restrictions are passed by default. A dialed number that has neither failed a restriction nor passed all restrictions explicitly will put the restriction server in a "parsing state". Since restrictions have not been failed explicitly, a user would still have a clear speech path to the CO. This situation thus provides a dialing "fraud window" to a perpetrator.
Fraudulent use of the telephone network has been prevalent over the years and a large amount of activity by network service providers has been directed at minimizing the effectiveness of the fraud perpetrator. Most of the fraudulent activity has been centered on the use of a "blue box" and/or "black box".
The use of a blue box is based on the situation whereby a caller pays for a local call while actually making a toll call. The blue box is basically the combination of a DTMF generator and a single-frequency generator operated manually in response to perceived single frequency signaling on the toll network. On the other hand, the use of a black box is based on "fooling" the toll billing system in believing that a toll call attempt was not successful whereas in actual fact, the call was answered. Whatever the equipment or method, a fraudulent call involves either avoiding or altering the billing record and charges for the call.
Many of the activities directed at mitigating if not altogether preventing the fraudulent use of the toll telephone network have resulted in numerous patents.
For example, U.S. Pat. No. 4,001,513 issued to the Northern Electric Company relies on the recognition of extraneous supervisory signals, generated in response to the use of a blue box, to detect a fraudulent toll call. U.S. Pat. No. 4,885,767 followed a similar detection technique to detect fraudulent calls.
U.S. Pat. No. 4,811,378 teaches an invention for detecting the results of using a blue box by monitoring the signaling activity in a toll office. A toll call is prevented from being set-up until answer supervision is received for a first attempted call.
Other detection of fraudulent practices in the toll network are described in U.S. Pat. Nos. 5,392,335 and 5,602,906; the methods vary respectively from the prevention of fraudulent toll calls that may be perpetrated through the use of telephone system adjunct processors such as voice messaging systems, to the detection of fraudulent toll calls using artificial intelligence and expert systems that detect "uncharacteristic" use of billing numbers.
The common characteristic of the known patents that teach the detection of blue or black boxes is that they rely on the effects of using those boxes; that is, they are directed at the detection of "non-normal" toll network signals that result from their use. None of the known patents teach a method for preventing the use of a hand-held DTMF generator to perpetrate fraudulent toll calls.
It is therefore an object of the invention to provide a method for mitigating the effective use of a hand-held tone generator by a subscriber to place fraudulent toll telephone calls.
It is a further object of the invention to prevent a key set user of a key telephone system from using a hand-held tone generator to place fraudulent toll calls.